Think of your 02 sensors like this...basically a fuel / air mixture goes into the engine, then the spark causes it to ignite and then it goes out of the exhaust and past the 02 sensors and out to the exhaust tailpipe.
So ...if you have say a bad spark plug or faulty Ign leads (there are no sensors for these) or in fact any other of a number of faults. The fuel that goes into the effected cylinder doesn't ignite properly and therefore raw fuel goes out through the exhaust, the 02 sensors pick this up as a fault saying that bank is running too rich (or too lean as the case may be) and set an 02 sensor code. Out you go and replace the sensors when in fact it is really nothing to do with them the problem is before it gets to them. In this situation you really have to go back to "the old school" the days before sensors. Look at it like an old fashioned car and start with the basics, in what condition are your plugs, wires etc etc. Many things can cause 02 sensor codes, too many to list on here but in your case it appears that both banks are effected (left hand and right hand bank) therefore you are either looking for bad plugs, wires on both banks or something that effects both sides of the engine.
There are a lot of smart LR people on here and many of us could throw good ideas at you but at the end of the day it is very hard to diagnose problems without seeing the vehicle and knowing the history etc. My advice is find a good local LR specialist / Dealer and get them to diagnose it for you, sometimes a good LR tech can open the hood and tell you in a second or two as they spot it straight away or have run into so many times. Think about it, maybe you didn't have to replace those 02 sensors? how much did that cost? sometimes it can be cost effective to have a pro look at it first.
